Assistant Professor, Soybean Extension Agronomist, Iowa State University

Weeds were the most yield limiting variable in Iowa in 2009. While this cannot be scientifically proven, the statement is based on anecdotal observations around the state; weedy fields were more common than historically observed. And honestly, this trend is getting worse. The last two years have been two challenging years for soybean producers in Iowa. We have experienced very wet conditions and the first application of glyphosate has been delayed meaning that weeds were larger that we would like to see when the fields got sprayed. Extension Weed Specialists at Iowa State University are in agreement with the Extension agronomists. The importance of managing weeds vs. controlling weeds have often been ignored after the widespread adoption of genetically modified (GM) crops, such as glyphosate-resistant soybean, since the system is presumed to be so efficient. However, this may (hopefully) be changing now.

Weed management is considered to be the most yield limiting crop pest by growers when compared to diseases, insects and nematodes. Weed management systems available in GM crops, primarily glyphosate applied alone, have many attractive attributes including the control of a broad spectrum of weeds, flexible application timing, relatively low cost of the herbicide, lack of crop phytotoxicity, and the perceptions of convenience and simplicity. However, the rapid adoption of glyphosate-resistant crops and grower weed management decisions to use glyphosate recurrently without due consideration of integrated weed management and stewardship has resulted in the evolution of glyphosate-resistant weed biotypes despite suggestions that resistance to glyphosate would never evolve.

In 2009, the evolution of glyphosate-resistant weed populations in Iowa confirmed from in-field experiments. Specifically, cases of glyphosate-resistant common waterhemp and giant ragweed that are two of the most yield-robbing weeds for Iowa soybean producers. It has not been confirmed yet that it is a widespread problem in Iowa but time to react is now since it can be a widespread problem very soon.

Controlling weeds early is extremely important for successful soybean production. The problem is we often do it too late. Soybeans are not strong competitors with weeds in the early part of the season and weeds will reduce yield because they compete directly with soybean for nutrients, moisture and light. They will also harbor insects and many diseases. Finally, they can also interfere with harvest, causing extra wear on harvest equipment or loss of yield due to the inefficiencies they cause with the sieves in the combine. Soybean yield losses resulting from weed interference and the cost of weed control do not constitute some of the highest costs involved in soybean production anymore – yes, that is true and that may be the problem. If you spend a lot of money on something you want to be sure that it works and that you get your money back.

The discovery of glyphosate-resistant weeds in Iowa should be a wakeup call to all of us. When something is so good it is often “too good to be true”. There will always be an end to it. We lost a lot of yield from weeds in 2009 and I do not see that it will be less in 2010 if we do not start to change our weed management systems and practices.

Maximum yields are not possible when weeds are present. The high yield soybean research plots are treated with a pre-emergence herbicide and then again with a minimum of one postemergence application, simply because yield protection is critically important. The pre-emergence herbicide helps mitigate the risk of weed competition if an extended wet period occurs since weeds always grow regardless of the weather. Adoption of glyphosate-resistant crops is likely the greatest global man-caused biological experiment in the six millennia of agriculture. Let’s try to protect this critically important crop trait. Resistant weeds can be managed successfully but it requires changes. It is time!

-Palle Pedersen, Associate Professor, Soybean Extension Agronomist, (palle@iastate.edu)

* Contributions by Micheal D.K. Owen, Professor, Weed Science Extension (mdowen@iastate.edu) Iowa State University

** Content Funded Through Soybean Checkoff